The ratio of the ordinates of Fig. Ic to Fig. Ib were calculated to determine the behavior of the nozzle reflected wave propagating from the nozzle to the injector. The ratio of the wave slope at the injector to the wave slope at the nozzle is always greater than unity, indicating wave-steepening for all waves propagating upstream against an accelerating gas flow. The occurrence of wavelets behind the initial wave and the subsequent coalescence of these wavelets to produce a smooth fronted wave near the nozzle entrance have been verified previously.5 It is assumed that these wavelets occur due to the increased evaporation rate and subsequent combustion of the fuel in an oxidant-rich environment. Some of the additional mass-energy is used to drive the wave, and part is available to increase the chamber base pressure. Figure 2 shows the ratio of base pressure at the injector at the arrival of the nozzle reflected wave to the pressure at that point prior to the incident wave. The data indicate that pressure amplification is inversely proportional to the chamber length and maximum slope of the steady-state pressure curve. The amount of wave-initiated evaporation, coalescence, and subsequent increase in chamber base pressure is greater with showerhead injectors than with impinging injectors. This may be related to the fuel drop size in the chamber. In the impinging injector motors, the fuel drop size is smaller, and the conversion of liquid propellants to gaseous products is more rapid than in the showerhead injector motors. Therefore, there is less mass and energy available to drive the waves or increase the base pressure in the impinging injector motors. For all injectors tested, the input waves tend to be completely attenuated, and the base pressure assumes its prewave value after three traverses of the chamber. At this point, however, a second train of longitudinal waves occur in engines using showerhead injectors. These waves all have rise times of 50 jitsec, independent of the chamber length. The frequency of the disturbance (defined as the reciprocal of the time between two successive waves traveling in the same direction) corresponds to the fundamental longitudinal Mach number compensated mode.5 The presence of secondary oscillations is not found with impinging injector motors. In conclusion, it appears that wave-steepening and pressure amplification are strongly coupled to the steady-state gasdynamic flow field through which the wave must propagate. Those injector-chamber configurations that result in rapid propellant utilization and high pressure gradients tend to inhibit wave growth. Less rapid conversion to gaseous products, with an attendant low pressure gradient, provides an energy and mass source to drive the wave and amplify the base pressure.